The present invention relates to a vertical MOSFET formed in a trench formed in a major surface of a semiconductor substrate, and a method of manufacturing the same.
A conventional vertical MOSFET (Metal Oxide Semiconductor Field Effect Transistor) has a compact structure but provides a large current drive capability as compared with a conventional lateral MOSFET. A typical example of a conventional vertical MOSFET is illustrated in FIGS. 1 and 2. Referring to FIG. 1, an n-type layer 2 is formed on a major surface of a p-type silicon substrate 1. A V-groove 3 is formed to be deeper than the n-type layer 2 and reaches the p-type silicon substrate 1. The V-groove 3 has an inverted quadrilateral pyramid shape, as indicated by the broken lines of FIG. 2. A gate oxide film 4 is formed in the V-groove 3. A gate electrode 5 is then formed on the gate oxide film 4. A p-type layer 6 is formed in a surface layer of the n-type layer 2 around the V-groove. Although omitted in FIG. 1, an Al electrode wiring layer connected to the p-type layer 6 through an insulating film is formed on the substrate surface. An electrode wiring layer connected to the gate electrode 5 is also formed. Referring to FIG. 2, reference numeral 19A denotes a contact hole formed in the insulating film so as to electrically connect the p-type layer 6 to an Al electrode wiring layer 20A indicated by a hatched region. A dotted region 16A represents a gate electrode including the gate electrode 5 and the wiring layer 20A formed integrally therewith. A region 14A of FIG. 2 corresponds to the V-groove 3 of FIG. 1. A region 18A surrounded by an alternate long and short dashed line represents an element formation region.
In the structure described above, the p-type layer 6 and the p-type silicon substrate 1 constitute source/drain regions, respectively. It is difficult to form a contact at the bottom of the V-groove. For this reason, the conventional VMOSFET can be used only when the contact for the source/drain region at the bottom of the V-groove is not required, i.e., only when the source/drain regions are used as a common terminal.
If an electrode of the source/drain region in the bottom of the V-groove is formed, a contact hole must be formed in a region excluding the V-groove region. For this reason, the transistor area is increased when viewed from the top, resulting in inconvenience.
Before assessing current drive capabilities of conventional vertical and lateral MOSFETs, i.e., magnitudes of currents flowing through the source-drain paths thereof, a structure of a typical conventional lateral MOSFET will be briefly described. As shown in FIG. 3, a gate electrode 16B is connected to a gate region (not shown) through a gate insulating film (not shown). Al electrode wiring layers 20B constituting source/drain electrodes are electrically connected to corresponding source/drain regions through contact holes 19B. The lateral MOSFET does not have the V-groove 3 shown in FIG. 1. Reference symbol W denotes a channel width.
Now the current drive capabilities of the conventional vertical and lateral MOSFETs are assessed. In this case, a minimum pattern size and an overlay accuracy are given as 1 .mu.m and 0.5 .mu.m, respectively. Minimum element formation regions of the vertical and lateral MOSFETs are 2.times.4.5 .mu.m.sup.2 and 2.times.6 .mu.m.sup.2. In this case, the effective channel width W (FIG. 3) of the lateral MOSFET corresponds to the length of the gate electrode 16A and is 1 .mu.m. The effective channel width of the vertical MOSFET is a length around the V-groove 3 (FIG. 1). Since a length of the upper portion of the V-groove 3 is different from that of the bottom portion thereof, normally, the effective channel width is given at an intermediate depth as indicated by arrows. For example, as shown in FIG. 2, a length at an intermediate depth between the square opening having a 1-.mu.m side and the vertex of the inverted quadrilateral pyramid as the effective channel width is given to be 0.5 .mu.m.times.4=2 .mu.m. The current drive capabilities of the FETs are proportional to the effective channel widths when their other characteristics are identical and their effective channel lengths are equal. The current drive capabilities of the conventional vertical and lateral MOSFETs are given to be as low as 0.25 and 0.17 when the effective channel widths are standardized per unit element formation area.